Effect of changes in C3 and C4 vegetation on isotopic signature of terrestrial carbon fluxes

When climate turns from an ice age to an interglacial the global biogeography undergoes major changes. During the present interglacial, i.e. the Holocene, the boreal tree line has moved further north and due to a less intense monsoon system North Africa is today not any more as green as during the last ice age. These changes must have affected also the distribution of C 3 and C 4 plants, and accordingly the isotopic carbon fractionation. This shift in isotopic signature of terrestrial carbon fluxes might have contributed to changes in atmospheric d13CO 2 over the Holocene as reconstructed from ice cores. In a simulation study based on the land component JSBACH of the Max Planck Earth System Model (MPI-ESM) we test the possible range of d13C changes due to C 3 /C 4 differentiation. JSBACH comprises a module for dynamical vegetation and disturbances by wind and fire. It can be run offline driven by climate parameters obtained from observations, or reconstructions, or online directly coupled to ESMs of full or intermediate complexity. To analyze the possible d13C changes a module to calculate carbon fractionation was implemented. The effective fractionation processes for C 3 and C 4 plants are based on Lloyd and Farquhar (1994) with an extension to include leaf- and photorespiration after Wingate et al. (2007). To estimate the possible range in d13C due to changes in C 3 and C 4 coverage, we present simulations for carbon fractionation in a pure C 3 and a pure C 4 land biosphere for present day climate taken from the CMIP5 model inter comparison